The known construction of condensing furnaces is one which typically requires the combustion gases to be passed first through a heat exchanger and then through a fin tube condenser which acts as a heat exchanger between the hot gases resulting from combustion and the air being drawn into the furnace at the suction of the air blower. Normally, the fin tubes are either vertical or sloped downwardly in the direction of the gas travel, so that liquid condensate may collect at the bottom in a manifold or the like, from where the condensate may be tapped off and discarded.
Typically, a condensing furnace which utilizes natural gas as fuel will produce a liquid condensate having a pH in the region of 3.8 to 4.8. By using a good grade of stainless steel for the portions contacted by this acidic condensate, damage can be kept to a minimum. However, when a condensing furnace burns oil, the condensate resulting from the hot products of combustion tends to be more acidic, having a pH in the range of 2.4. At this level of acidity, the condensate tends to attack even good grades of stainless steel, and particularly the weld locations between the tubes and the manifolds at either end.